The present invention relates to a semiconductor laser element and a process for fabricating it.
To fabricate semiconductor laser elements easily, a structure called a ridge waveguide type is often used. FIG. 1 is an illustrative schematic of how to prepare that structure. First, an n-type cladding layer 102, an active layer 103, a p-type cladding layer 104 and a p-type contact layer 105 are grown on a substrate 101. Then, a striped shaped resist is formed on the surface of the wafer by photolithographic patterning. Following this, the resist is used as a mask to wet etch the wafer in such a way that the p-type cladding layer is left with only a desired thickness, thereby forming a striped form of ridge. By use of this ridge structure, it is possible to stabilize the lateral mode of operation and reduce threshold currents in laser oscillation.
The fundamental properties of the thus prepared semiconductor laser element are determined by the thickness d.sub.p (106) of the remaining cladding layer and the ridge width W (107). For instance, if d.sub.p becomes too thick, there is then a threshold current increase, which will be less effective for lateral light confinement. In contrast, if d.sub.p becomes too thin, there is then a working current decrease, which will make lateral light confinement more intense. At a large W the laser beam diverges very little in the lateral direction, whereas at a small W the laser beam diverges appreciably. To obtain desired semiconductor lasers in good-enough yields, it is necessary to place these two values under strict control. In conventional processes, however, the value of d.sub.p has a large distribution within the plane of the wafer, because both the uniformity of the film thickness of the grown crystal and the accuracy of etching depth pose problems in the formation of a DH structure.
As illustrated in FIG. 2, there is a twin-striped (TS) laser that is a modified ridge type laser. Upon controlled in terms of the lateral mode of operation, this TS laser is known to have useful characteristics such as current-light non-linearity characteristics and far field beam deflection. So far, TS lasers have been provided with two sets of ridges by wet etching in conventional manners. With this procedure, however, difficulty was involved in placing not only the above-mentioned d.sub.p (201), W.sub.1 (202) and W.sub.2 (203) but also a distance, t (204), between two stripes and an etching depth of striped groove (205) under precise control. These TS laser characteristics are closely correlated with the symmetry of the two ridges. Without having a precise understanding of these structural parameters, it is impossible to extract excellent characteristics form TS lasers.
As a result of strenuous studies, the inventors have now found that such a problem arises from the insufficient uniformity of the p-cladding layer and insufficient accuracy of etching depth. Thus, it has now been discovered that the thickness d.sub.p of the remaining cladding layer cannot only be controlled more accurately but the relative arrangement of the ridges can be kept in good condition as well by doing the control of d.sub.p without recourse to etching, resulting in the achievement of stable and improved laser characteristics. This discovery underlies the invention.